The present invention relates to a brake system with electronic brake force distribution according to the preamble of claim 1.
A brake system of this type is disclosed in German patent application No. 43 37 498. The prior art brake system controls the brake force applied to the rear wheels in relation to the brake force applied to the front wheels in order to ensure ideal brake force distribution. Pressure control valves are arranged in a hydraulic circuit which connects the master cylinder of the brake system to the rear-wheel brakes. There is provision of an auxiliary power source to supply an outlet pressure and a hydraulic booster to modulate the braking pressure and to actuate the master cylinder. A change-over valve is interposed between the master cylinder and the control valve and selectively movable into a first operating position connecting the control valves to the master cylinder and closing the connection between the control valves and the hydraulic booster, and into a second operating position connecting the control valves to the hydraulic booster and closing the connection between the control valves and the master cylinder. When the change-over valve is shifted from the first operating position to the second operating position, the control valves will control the hydraulic pressure in the rear-wheel cylinders corresponding to the predetermined relation to the front-wheel braking pressure. The commencement of this control depends upon whether or not a defined start condition is satisfied. Defined criteria must be given: The brake light switch must be activated, the vehicle speed must have exceeded a determined threshold value and the vehicle acceleration must have fallen short of a determined negative threshold value. Finally, the wheel speed of the rear wheel which necessitates braking pressure control must fall short of the wheel speed of the front wheel that lies in front thereof by a defined amount. If these conditions are satisfied, electronic brake force distribution will set in. Distribution is discontinued as soon as either the brake light switch is deactivated (which means that the driver has removed his foot from the brake pedal) or the vehicle acceleration exceeds a determined negative threshold value which does not have to be identical to that one of the start conditions.
Electronic brake force distribution should only be effected when it is needed. Therefore, a deactivation criterion which is irrespective of the position of the brake light switch is certainly an expedient supplementation. However, calculating the vehicle acceleration at very low wheel speeds involves a large number of errors because wheel sensors at low speeds do not issue exact data about the vehicle velocity. Besides, there are brake systems with start conditions for the entry into electronic brake force distribution which are independent of whether or not pedal-operated braking exists. This means that, under certain circumstances, electronic brake force distribution is initiated when driving through a road hole or caused by any other road surface disturbance which took effect on the speed of a single wheel. Finally, it is still possible that malfunction of a brake light switch is not detected, or that it is not recognized due to malfunction of a brake light switch that the driver has already terminated pedal-operated brake application.
To ensure deactivation of the electronic brake force distribution and to avoid unnecessary valve actuation even in such cases, an object of the present invention is to achieve a brake system of the above-mentioned type which triggers deactivation of the electronic brake force distribution independently of the application of the brake pedal and irrespective of exceeding a defined vehicle acceleration in the absence of need for pressure reduction at the rear-wheel brakes.
This object is achieved in connection with the characterizing features of claim 1. The principle of the present invention includes recognizing that the driver, in spite of continuing braking, does not have the intention to decelerate greatly. One may consider this assumption given if a braking situation lasts longer than 7 seconds, for example, which situation necessitates neither reducing the braking pressure nor maintaining the braking pressure on the rear axle but, on the contrary, would easily be able to generate a still greater brake torque.
A second condition may be applicable in parallel, and the electronic brake force distribution will then be discontinued as soon as any one of the two conditions is satisfied. This second condition may then be the exceeding of a defined negative vehicle acceleration.
In parallel to this second condition, or in addition thereto, it can be checked whether pedal-operated braking continues to exist. Because the identification of pedal-operated braking depends on the position of a brake light switch, it is advisable to integrate brake light switch error detection into the brake system to prevent premature termination of the electronic brake force distribution due to a defective brake light switch.